1. Field of the Invention
The present invention relates to a common rail mounted in a pressure accumulation fuel injection device for accumulating high-pressure fuel.
2. Description of Related Art
A pressure accumulation fuel injection device is known as a fuel supply device of an internal combustion engine such as a diesel engine for pressurizing fuel suctioned from a fuel tank with a pump and for supplying the fuel into combustion chambers of respective cylinders of the engine from injectors through injection. The pressure accumulation fuel injection device has a common rail for accumulating high-pressure fuel discharged by a fuel supply pump. The pressure accumulation fuel injection device distributes the high-pressure fuel accumulated in a pressure accumulation chamber of the common rail to multiple injectors mounted in the respective cylinders of the engine and injects the fuel into the combustion chambers of the respective cylinders of the engine from injection holes formed in axial tip ends of the injectors.
An example of conventional common rail is shown in FIG. 20. The common rail 201 is a pressure accumulation vessel for accumulating high-pressure fuel pressure-fed from a high-pressure fuel pump such as a supply pump. The common rail 201 is formed with a pressure accumulation chamber (center hole) 223 for accumulating the high-pressure fuel inside. The common rail 201 has a pipe joint 221 formed with an external screw 225 on an outer peripheral face thereof. An external pipe such as a high-pressure pump pipe or an injector pipe is connected to the external screw 225. A central portion of the outer end of the pipe joint 221 communicates with the pressure accumulation chamber 223 through an inside-outside communication hole 224.
The inside-outside communication hole 224 is formed with an orifice α for reducing pressure pulsation accompanying an injection operation of an injector or pressure pulsation accompanying a pressure-feeding operation of the high-pressure fuel pump. The conventional orifice α is provided by forming a hole directly in a main body 220 (rail main body) of the common rail 201. Because of restrictions related to hole making process, the orifice α is formed at the bottom of the inside-outside communication hole 224. As shown in FIG. 20, the orifice α opens into the pressure accumulation chamber 223.
Since the high-pressure fuel is accumulated in the pressure accumulation chamber 223, the high pressure acts on an inner peripheral face of the pressure accumulation chamber 223. The orifice α having a small diameter opens in the inner peripheral face of the pressure accumulation chamber 223 while the orifice α crosses with the inner peripheral face. Hereinafter, the opening of the orifice α, at which the orifice α crosses with the inner peripheral face, is referred to as a crossing hole. As the crossing hole decreases, greater stress is concentrated in an opening edge of the crossing hole. Therefore, the common rail 201 with the orifice α formed integrally in the rail may body 220 by the hole making process is used in a pressure accumulation fuel injection device using relatively low-pressure accumulation value of the pressure accumulation chamber 223 (180 MPa or lower, for example).
In recent years, aiming to improve exhaust characteristics and the like, increase of the common rail pressure over 180 MPa has been required. However, since the crossing hole of the orifice α is small in the common rail 201 with the orifice α formed integrally in the rail main body 220 by the hole making process, it is difficult to ensure a safety margin related to fatigue strength.
Aiming to ensure the safety margin related to the fatigue strength, a proposed common rail 201 has a separate bush that is separate from the rail main body 220 and that is formed with an orifice α instead of forming the orifice α directly in the rail main body 220. The bush is press-fitted to an inside of the inside-outside communication hole 224. Thus, the crossing hole is enlarged (for example, as described in JP-A-2001-82663 or JP-A-2001-280217.
The conventional technology of press-fitting the bush formed with the orifice α to the inside of the inside-outside communication hole 224 press-fits the outer peripheral face of the orifice α into the inside-outside communication hole 224. There is a possibility that the bush receives a differential pressure between the pressure in the pressure accumulation chamber 223 and the exterior pressure. Therefore, in order to prevent the bush from coming off of the inside-outside communication hole 224, the bush is tightly press-fitted to the inside of the inside-outside communication hole 224.
Therefore, there is a possibility that an inner diameter of the orifice α is changed by distortion caused by the press-fitting. If the inner diameter of the orifice α changes, designed passing of the fuel is disturbed. As a result, there is a possibility that injection characteristics of the injector change and designed injection cannot be performed.
The bush formed with the orifice α is press-fitted into the inner periphery of the external screw 225 of the pipe joint 221. Since the bush is tightly press-fitted to the inside of the inside-outside communication hole 224, there is a possibility that the external screw 225 formed on the pipe joint 221 is deformed by the distortion caused by the press-fitting. IF the external screw 225 is deformed, there is a possibility that a trouble is caused in screwing of a pipe nut for fixing the external pipe to the joint 221.
Another example of common rail mounted in the pressure accumulation fuel injection device has a substantially cylindrical rail main body, in which a pressure accumulation chamber for accumulating the high-pressure fuel inside is formed in a longitudinal direction (axial direction). The rail main body is formed with multiple inside-outside communication holes for connecting the pressure accumulation chamber with the outside. Out of the multiple inside-outside communication holes, the inside-outside communication hole provided upstream of the pressure accumulation chamber with respect to a flow direction of the fuel communicates with the discharge hole of the fuel supply pump through a high-pressure pump pipe. The other multiple inside-outside communication holes provided downstream of the pressure accumulation chamber with respect to the flow direction of the fuel communicate with the insides of the injectors through multiple injector pipes.
The fuel supply pump incorporates a plunger driven by a cam to linearly reciprocate inside the fuel supply pump. Thus, the high-pressure fuel is intermittently discharged from the discharge hole of the fuel supply pump into the pressure accumulation chamber through the high-pressure pump pipe in a predetermined cycle. Accordingly, the high pressure is generated in the high-pressure pump pipe in a pulsating manner in accordance with the shape of the cam. The pressure pulsation (discharge pulsation of the fuel supply pump) is propagated to the inside of the pressure accumulation chamber as a pressure wave.
The multiple injectors connected with the common rail open intermittently at different injection timings to perform the fuel injections. The pressure in the injector pipe temporarily decreases when the injector opens. Therefore, pressure pulsation of the high pressure and the low pressure is generated in the injector pipe. The pressure pulsation is propagated to the inside of the pressure accumulation chamber as a pressure wave (reflection wave generated in accordance with opening and closing of the injector).
In the pressure accumulation chamber of the common rail, the pressure wave from the fuel supply pump merges with the reflection waves from the injectors. Therefore, even during a constant operation, the fuel pressure in the pressure accumulation chamber of the common rail is not constant pressure but fluctuates. The pressure pulsation affects valve opening timing, valve closing timing and fuel injection pressure of the injector of the same cylinder or the other cylinder. As a result, the injection timing and the fuel injection amount vary and a difference is caused in the injection amount among the cylinders.
Therefore, conventionally, orifices (fixed restrictors) are provided in the inside-outside communication holes of the rail main body of the common rail or fuel passages of pipe connectors fluid-tightly connecting the injector pipes with the rail main body of the common rail. Thus, propagation of the reflection wave, which is generated by opening and closing of the injector in a certain cylinder, to the inside of the pressure accumulation chamber of the common rail is inhibited to reduce the influence on the fuel injections in the other cylinders. In addition, the reflection wave generated by the opening and closing of the injector of the certain cylinder is damped to reduce the influence on the next injection in the same cylinder.
However, in the conventional common rail, there is a manufacture variation in an orifice diameter of the orifice, which is provided in the inside-outside communication hole communicating with the inside of the injector of each cylinder of the engine or in the fuel passage of the pipe connector. The propagation of the reflection wave, which is caused by the opening and closing of the injector, to the inside of the pressure accumulation chamber of the common rail cannot be prevented sufficiently by only providing the orifice in the inside-outside communication hole or the fuel passage.
A common rail aiming to damp a reflection wave from an injector of a certain cylinder and to eliminate an influence on next injection in the same cylinder and fuel injection in another cylinder is described in JP-A-2001-207930. In this common rail, an orifice is formed in a piston capable of sliding in the inside-outside communication hole of the rail main body of the common rail or the fuel passage of the pipe joint. The piston follows the pressure pulsation in the rail main body and the reflection waves from the injectors to damp the pressure pulsation in the rail main body and the reflection waves from the injectors. A first spring is provided upstream of the piston with respect to the fuel flow direction and a second spring is provided downstream of the piston. An end of the piston provides a first spring seat portion for receiving a spring load of the first spring and the other end of the piston provides a second spring seat portion for receiving a spring load of the second spring.
In this common rail, the orifice is formed to penetrate through the entity of the piston in the axial direction. Therefore, a process length of the orifice is long. A process time of orifice forming process requiring highly accurate processing technology is lengthened. As a result, a cost is increased. This common rail requires two springs (first and second springs). Therefore, the number of the parts is increased, increasing a cost. In this common rail, selection of spring constants of the first and second springs for damping the pressure pulsation and the reflection waves is difficult. For example, it is difficult to decide which spring constant should be increased out of the spring constants of the first and second springs. Therefore, the pressure waves (discharge pulsation of fuel supply pump and reflection waves from injectors) significantly affecting the injection amount characteristics (injection timing, injection amount, injection ratio and the like) of each cylinder of the engine cannot be sufficiently restricted to be small.
The influence of the pressure pulsation inside the pressure accumulation chamber of the common rail on the valve opening timing, the valve closing timing and the fuel injection pressure of the same cylinder or the other cylinder cannot be eliminated. As a result, the difference in the injection pressure or the injection amount among the cylinders cannot be sufficiently restricted to be small.